1. Field of the Invention
The present invention relates to an electromagnetic switch having a variable magnetic resistance and, more particularly an electromagnetic switch suitable for a starter for starting an engine.
2. Related Art
Various electromagnetic switches are used in conventional starters. The starter disclosed in JP-A 50-5807, for instance, has a pinion gear which is helical spline-fitted on the outer periphery of an output shaft and a rotation restricting member which restricts the rotation of the pinion gear. The rotation restricting member is driven by the attraction force of an electromagnetic switch at the time of a start of the rotation of an armature thereby to restrict the rotation of the pinion gear, so that the pinion gear is moved axially by the thrust force generated by the helical spline to engage with the ring gear of an engine.
According to this rotation restricting type, in comparison with the type in which the pinion gear is pushed out by the attraction force of the electromagnetic switch through a drive spring (spring force characteristics line C2 in FIG. 4) to engage with the ring gear, the spring force for operating the electromagnetic switch can be decreased (spring force characteristics line C1 in FIG. 4). As a result, the attracting force (required attracting force) of the electromagnetic switch can be decreased correspondingly, and the size and weight of the electromagnetic switch can be advantageously reduced.
In the above starter of the type which restricts the rotation of the pinion gear, the decrease in the spring force is so large that the attracting force characteristics of the electromagnetic switch exceeds greatly the spring force as the air gap in the electromagnetic switch becomes smaller. The attraction force of the plunger of the electromagnetic switch rises too high, causing the movable contact attached to the plunger to bounce at the time of abutting the fixed contact. As a result, due to the chattering (repetition of engagement and disengagement) of the movable contact, the life of the contact is shortened.